1. Field of the Invention
The present invention relates to non-impact printing apparatus and method for recording using printheads having a plurality of recording elements such as light-emitting diodes (LED's) or the like.
2. Brief Description of the Prior Art
In U.S. Pat. No. 4,074,319 a printhead employing linear arrays of LED's for recording is described. Light from selective illuminated LED's impinges upon a photosensitive surface such as a photographic film or electrostatically charged photoconductor to form a latent image. This image may be developed by conventional known means to form a visible image. In forming images that vary in density it is further known to adjust the size or density of each picture element (pixel) recorded by each LED in steps so that certain pixels may be said to have greater grey levels than other pixels. In recording a grey level pixel, the pixel may be recorded for a single timed duration that varies with a signal representing this time duration as, for example, disclosed in U.S. application Ser. No. 290,002. Another approach disclosed in U.S. Pat. No. 4,074,319 employs a series of rapid flashes to record a single pixel wherein flashes may be either of uniform duration or of varying or binary weighted duration.
The invention described herein relates to the latter type of grey level recording wherein each pixel is recorded using a series of pulses, except of course for those pixels requiring only a minimum of exposure by only a single flash of the respective LED. The invention further relates to the recording of such pixels using a binary architectured printhead. As this term is used herein, it relates to known types of printheads wherein driver chips are provided with shift registers and latches for storing a single data bit for driving each LED for producing a single flash of light. The driver chips conventionally also include current driving circuitry for driving current to those LED's to be flashed during a period of time controlled by a strobe pulse timed by a suitable clock or by the presence of a data bit stored in a latch for driving the LED. While the LED is being driven or flashed in response to a data bit stored in the latch, a shift register on the driver chip may be receiving the next data bit for driving respective LED's for the next flash.
The binary architectured printheads may be distinguished from grey level architectured printheads wherein the driver chips of the latter store the entire multibit signal for recording a pixel.
The desirability of using binary architectured printheads for grey level recording lies in their cost which is significantly less expensive than more complex grey level architectured printheads.
In U.S. application Ser. No. 07/521,511, U.S. Pat. No. 5,138,337 a binary architectured printhead is disclosed for recording grey level pixels. In employing such printheads, data needs to be formatted appropriately before sending individual data bits to the printhead. In formatting the data to the printhead, typically the data will first be stored in a page memory as a multibit grey level signal. The data is then extracted from page memory and may be required to be adjusted for process conditions as well as for correcting the data that needs to be reformatted for transmission to the printhead. A problem associated with the above where such adjustment and correction is required is the need for memory for the storing the data in the various stages.
It is an object of the invention therefore to minimize the memory requirements for manipulating data prior to sending same to a binary architectured printhead.